Plug the 4 small holes in the actual overflow. Use 2 of the remaining for large holes as intakes ( probably the back 2) and then the other 2 as the return (you have a return line bulkhead already setup. (That's the hole below the slots or weir).

In the middle of the tank you could either plug those with bulkheads and a plug or do an additional center overflow and return. (That one is more risky because if power goes out... all the water goes into it)

Big tanks are awesome!

Kathy, That's not always true. If one of those lines comes loose or the pump needs maintenance you need a more sure-fire way of preventing leaks ie. a ball valve.